2.14 Ventromedial Hypothalamus: Definition & Function
Definition and Location of the VMH
No, the ventromedial hypothalamus (VMH) is not a reptile, it’s part of the brain that deals with several body systems. This long scientific name can be broken into three component parts that mostly describe its location in the brain.
- ‘ventral’ is belly or front,
- ‘medial’ means near the middle, and
- ‘hypothalamus’ is a region of the brain just underneath the thalamus.
So the VMH is a portion of the hypothalamus gland in the brain, underneath the thalamus, near the middle of the brain, and towards the front of the hypothalamus. Got it? Well, at least that little exercise tells us where this brain part is, but what does it do?
The Functions of the VMH
Experiments in rats have shown the VMH to be involved in food satiety, temperature regulation, fear response, and sexual activity. This is one busy little area of the brain. Imagine what would be happening in our bodies without it! Let’s take a look at some of these functions.
When the VMH of rats were intentionally damaged in early experiments, the results were dramatic; the damaged rat became quite overweight, leading scientists to believe the VMH was responsible for the telling the rat when to stop eating. This led to the VMH being known as the ‘satiety center’. However, it turns out that appetite control is surprisingly complex. While the most important regulation does seem to come from the VMH, further experiments have revealed that other areas of the brain, endocrine system, and nervous system also affect appetite.
Now we know it’s not accurate to label the VMH as the ‘center’ of satiety, but it is the most important cog in the complex system that determines satiety. This new label doesn’t have quite the same ring to it as the old one, but it gets closer to the truth.
It might be a natural assumption that all ‘fight or flight’ responses involve the same mechanisms, but it turns out that not all fears are created equal. Experiments involving more rats showed similar behavioral and neural reactions from exposure to dominant rats or to cats. Although it was far from the only brain part involved in these fear reactions, the VMH plays an important role in proper reaction to a predator or a dominant member of the same species.
Rats with damaged VMHs showed fewer fear behaviors of this type – that is, they did not hide as effectively from either predators or dominant members of their own species. In the wild, a healthy VMH could mean the difference between life and death.
However, VMH is not responsible for all fear-based reactions. Experiments involving pain from external sources did not activate the VMH, nor did damaged VMHs lead to a change in avoiding external pain sources. In short, there must be other brain pathways that promote fear response to these types of stimulus.
When our internal body temperature gets out of a very narrow range, temperature sensors send signals to the VMH saying either ‘it’s too hot’ or ‘it’s too cold’.
For high temperatures, the VMH sends signals to expand the small blood vessels near the skin’s surface. This allows heat to escape through radiation from the skin. Sweat glands are also encouraged to expel sweat onto our skin. The evaporation of this liquid causes tremendous cooling – essential for early human survival and according to some sources, this ability helped our ancestors become the dominant species on the planet! The VMH also sends signals to muscles controlling body hairs to lay flat, which allows more airflow to cool the skin.
For low temperatures, those signals are all reversed: blood vessels are constricted near the skin surface, body hairs stand upright, and sweat glands are deactivated. In addition, the VMH sends signals to muscles that make them shiver, producing heat in the surrounding tissue.
Measurements of the neural activity of monkey ventromedial hypothalamus before and during sexual activity indicates this brain structure is highly involved in sexual arousal. However, it is not yet known how the VMH regulates arousal.
The ventromedial hypothalamus is a small piece of the hypothalamus in the brain with a large range of functions that include: sexual activity appetite suppression, fear responses, and regulation of temperature. To put it a different way, a person without a VMH would eat too much, respond inappropriately to an escaped lion, have trouble maintaining a comfortable skin temperature, and have a low libido.
The VMH is located near the left-right center of the brain and towards the front of the hypothalamus gland, giving it a good place to access these functions. For example, it’s one of the most important brain systems that regulate appropriate level of food consumption. It can also stimulate the blood vessels, sweat glands and body hairs to respond to temperature changes, as well as prompt certain fear responses and sexual arousal, though the latter two aren’t fully understood.